The documentation sources are written in DocBook, which is a markup language superficially
similar to HTML. Both of these
languages are applications of the Standard
Generalized Markup Language, SGML, which is essentially a language for
describing other languages. In what follows, the terms DocBook
and SGML are both used, but
technically they are not interchangeable.

DocBook allows an author to
specify the structure and content of a technical document without
worrying about presentation details. A document style defines how
that content is rendered into one of several final forms. DocBook
is maintained by the OASIS group. The official
DocBook site has good introductory and reference
documentation and a complete O'Reilly book for your online
reading pleasure. The NewbieDoc Docbook Guide is very helpful for
beginners. The FreeBSD Documentation Project also uses DocBook and
has some good information, including a number of style guidelines
that might be worth considering.